I did something similar once, basically beat up RaceA till the point where they only had one planet. Enter RaceB: Warmongering and bloodthirsty. They come into the system, ignore my fleet there and bLast the hell out of the Last of RaceA's worlds. Farewell RaceA.
I felt really bad about that, so I went after RaceB, swearing to bring them to justice for what they'd done (but it was perfectly ok for me to nearly destroy RaceA, because I was the one doing it
).